K j saunders (auth ) organic polymer chemistry an introduction to the organic chemistry of adhesives, fibres, paints, plastics and rubbers springer netherlands (1988)

1.4.1 Polymerization through functional groups In this type of polymerization, reaction proceeds between pairs of functional groups associated with two different molecules.. A polymeriza

ORGANIC POLYMER CHEMISTRY

ORGANIC POLYMER CHEMISTRY

AN INTRODUCTION TO THE ORGANIC CHEMISTRY

OF ADHESIVES, FIBRES, PAINTS, PLASTICS AND

RUBBERS Second edition

Department of Applied Chemical and Biological Sciences

Ryerson Poly technical Institute, Toronto

LONDON NEW YORK CHAPMAN AND HALL

First published in 1973 by Chapman and Hall Ltd

11 New Fetter Lane, London EC4P 4EE

Second edition 1988 Published in the USA by Chapman and Hall

29 West 35th Street, New York, NY 10001

© 1973, 1988 K J Saunders Softcover reprint of the hardcover 2nd edition 1988

J W Arrowsmith Ltd Bristol

ISBN -13:978-94-0 10-7031-7

All rights re~erved No part of this book may be reprinted, or reproduced

or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording,

or in any information storage and retrieval system, without permission in

writing from the publisher

British Library Cataloguing in Publication Data

Saunders, K J (Keith John)

Organic polymer chemistry: an introduction

to the organic chemistry of adhesives, fibres,

paints, plastics and rubbers.-2nd ed

1 Polymers and polymerization

This book is dedicated with gratitude to

my parents, Leonard and Marjorie Saunders, for their sacrifices in earlier years and to

my wife, Jeannette,for her steadfast encouragement

in recent times

CONTENTS

1 Basic concepts

4 Poly(vinyl chloride) and related polymers 90

5 Poly(vinyl acetate) and related polymers 113

12 Other aromatic polymers containing p-phenylene groups 265

PREFACE

This book deals with the organic chemistry of polymers which find nological use as adhesives, fibres, paints, plastics and rubbers For the most part, only polymers which are of commercial significance are considered and the primary aim of the book is to relate theoretical aspects to industrial practice The book is mainly intended for use by students in technical institutions and universities who are specializing in polymer science and by graduates who require an introduction to this field There are available several books dealing with the physical chemistry of polymers but the organic chemistry of polymers has not received so much attention In recognition of this situation and because the two aspects of polymer chemistry are often taught separately, this book deals specifically with organic chemistry and topics of physical chemistry have been omitted Also, in this way the book has been kept to a reasonable size This is not to say that integration of the two areas of polymer science is undesirable; on the contrary, it is important that the inter-relationship should be appreciated

tech-I was gratified by the favourable comments prompted by the first edition of the book and I have therefore retained the same organization in this second edition Nevertheless, the book has been extensively revised to reflect the developments which have taken place The most noticeable features of the period since the publication of the first edition have been the continued dominance of the same bulk commodity polymers and the appearance of several new speciality engineering thermoplastics I have aimed to be compre-hensive in scope and so both the well-established and the newer polymers are dealt with in this edition

KJ.S

Department of Applied Chemical ana Biological Sciences, Ryerson Poly technical Institute, Toronto, Ontario, Canada

1

BASIC CONCEPTS

1.1 DEFINITIONS

A polymer may be defined as a large molecule comprised of repeating

structural units joined by covalent bonds (The word is derived from the Greek: poly - many, meros - part.) In this context, a large molecule is com-monly arbitrarily regarded either as one having a molecular weight of at least

1000 or as one containing 100 structural units or more By a structural unit is meant a relatively simple group of atoms joined by covalent bonds in a specific spatial arrangement Since covalent bonds also connect the structural units to one another, polymers are distinguished from those solids and liquids wherein repeating units (ions, atoms or molecules) are held together by ionic bonds, metallic bonds, hydrogen bonds, dipole interactions or dispersion forces

The term macromolecule simply means a large molecule (Greek:

macros -large) and is often used synonymously with 'polymer' Strictly speaking, the terms are not equivalent since macromolecules, in principle, need not be composed of repeating structural units though, in practice, they generally are

It may be noted that 'polymer' is often also used to refer to the massive state Then the term refers to a material whose molecules are polymers, i.e a polymeric material Likewise, the term resin is sometimes used to refer to any

material whose molecules are polymers Originally this term was restricted to natural secretions, usually from coniferous trees, used mainly in surface coatings; later, similar synthetic substances were included Now the term is generally used to indicate a precursor of a cross-linked polymeric material, e.g epoxy resin and novolak resin (See later.)

1.2 SCOPE

A great variety of polymeric materials of many different types is to be found throughout countless technological applications For the purposes of this book it is convenient to divide these materials according to whether they are

2 BASIC CONCEPTS

inorganic or organic and whether they are naturally occurring or synthetic Using this classification, the diverse nature and widespread application of polymeric materials is illustrated by Fig 1.1 This book is concerned solely with technologically useful organic polymeric materials These materials are commonly classified as adhesives, fibres, paints, plastics and rubbers accord-ing to their use Although these are all polymeric materials, they clearly possess a great diversity of properties about which few generalizations can be made It is significant, however, that no low molecular weight organic compounds are useful in the above applications The physical properties of

an individual polymeric material are largely determined by molecular weight, strength of intermolecular forces, regularity of polymer structure and flexi-bility of the polymer molecule

Adhesives Fibres Rubber

Fig 1.1 Some applications of polymeric materials

1.3 RISE OF THE CONCEPT OF POLYMERS

Nowadays the concept of polymers (or, simply, big molecules) is easy to accept but this has not always been the case The rise of the concept is of interest and a brief historical review is given below

By the 1850s the existence of atoms and molecules was accepted but mainly

in respect of simple inorganic compounds The application of these ideas to more complex organic materials was not understood so clearly At least, by this time the notion that organic compounds all contained a mysterious 'vital force' derived from living things had been finally abandoned as more and more organic compounds were synthesized in the laboratory

RISE OF THE CONCEPT OF POLYMERS 3

In 1858 Kekule suggested that organic molecules were somewhat larger than the simple inorganic molecules and consisted of atoms linked in chains

by bonds This led to the realization that the order in which atoms are arranged in the molecule is significant, i.e the meaning of 'structure' was appreciated These ideas, aided by improving methods of elemental analysis, resulted in the elucidation of the structure of many simple organic com-pounds such as acetic acid and alcohol However, virtually nothing was deduced about the structure of more complex organic materials such as rubber, cellulose and silk All that was clear was that these materials had elemental analyses which were quite similar to those of the simple com-pounds whose structures were known

The first significant information came in 1861 when Graham found that solutions of such natural materials as albumin, gelatin and glue diffused through a parchment membrane at a very slow rate Materials of this kind were called colloids (Greek: kolla-glue) In contrast, solutions of materials like sugar and salts diffused readily; these substances were called crystalloids

since they were generally crystalline The reason for this difference was not clear, but it was generally supposed that the colloid solute particles were rather large and therefore their passage through the membrane was hindered There were a few tentative suggestions that the colloids had high molecular weights so that a solute particle was large simply because it comprised one large molecule However, this view was not at all acceptable to most scientists

of the day At this time the current practices of organic chemistry demanded the preparation of crystalline compounds of great purity with exact elemental analyses and sharp melting points It was generally felt that if the colloids were 'cleaned up' they would crystallize and reveal themselves as 'normal' low molecular weight compounds This view was apparently reinforced by the fact that many inorganic materials of low molecular weight can be prepared

so that they behave as colloids, e.g colloidal arsenious sulphide, gold and silver chloride It was held, quite rightly, that in these cases the colloidal particles are aggregates of smaller particles held together by secondary valency forces of some kind (Incidentally, the physically associated groups were frequently called 'polymers' in the literature.) This view was very much

in keeping with the great emphasis which was being placed on van der Waals forces in the 1890s and early 1900s By analogy, the organic colloids were assumed to be molecular aggregates or micelles, a concept still to be found in the literature of the 1940s

The first worker to take a clearly opposite view was Staudinger in Germany in a paper of 1920 He maintained that the colloidal properties of organic materials are due simply to the large size of the individual molecules and that such macromolecules contain only primary valency bonds Staudinger's initial evidence was mainly negative Firstly, he demonstrated that the organic materials retain their colloidal properties in all solvents in

Such molecules were then supposed to aggregate by virtue of secondary valency forces arising from the presence of double bonds (Incidentally, the cyclic structure was held to account for the fact that no end-groups could be detected.) However, Staudinger showed that the hydrogenation of natural rubber produces a saturated material which still exhibits colloidal properties Thus he demonstrated that secondary forces originating from the unsatu-ration of natural rubber are unlikely to be responsible for colloidal behav-iour Staudinger proposed the long chain structures, which are accepted today, for several polymers Additional support for the existence of macro-molecules came with the development of methods of molecular weight deter-mination Until this time, only cryoscopic methods were available and these were inadequate for the very high molecular weights involved; also, it was commonly held that the laws which hold for ordinary solutions were not applicable to colloidal solutions

Thus by about 1930 the concept of polymers was firmly established even if not universally accepted The macromolecular viewpoint was finally secured largely by the work of Carothers in the USA This work was begun in 1929 and had as its objectives, clearly stated at the outset, the preparation of polymers of definite structure through the use of established reactions of organic chemistry and the elucidation of the relationship between structure and properties of polymers These researches were brilliantly successful and finally dispelled the mysticism surrounding this field of chemistry

One outstanding result of Carothers' work was the commercial ment of nylon Nylon stockings came on the market in 1940, when polymers,

develop-in terms of popular acceptance, might be said to have arrived The etical, practical and economic foundations had been laid and since this date progress has been phenomenal

theor-GENERAL METHODS OF PREPARATION OF POLYMERS 5

1.4 GENERAL METHODS OF PREPARATION OF

POLYMERS There are three general methods by which polymers may be prepared from relatively simple starting materials (monomers) Each of these methods is briefly described in this section; more detailed considerations are to be found throughout later chapters

1.4.1 Polymerization through functional groups

In this type of polymerization, reaction proceeds between pairs of functional groups associated with two different molecules Provided all the reacting molecules have at least two reactive groups, a sequence of reactions occurs and a polymer is formed It will be apparent that the structural units of the polymer will contain a group whose arrangement of atoms is not to be found

in the starting material A polymerization of this kind occurs when w-aminoundecanoic acid is heated:

or -HN-(CH2)\O-CO-HN-(CH2)\O-CO-HN-(CH2

)\O-CO-nH 2 N-(CH 2) 10 -COOH

1

The resulting polymer (nylon 11) consists essentially of a long chain of repeating -HN-(CH2)1O-CO- groups (the structural units); in a typical commercial material n has a value of the order of too The polymer contains amide groups (-CO-NH-) (which are not present in the starting material) at regular intervals along the chain and is, therefore, a polyamide It may be noted that in the representation of polymers it is common to leave unspecified the nature of the end-groups (as above) This practice is justified since the number of end-groups is very small compared to the number of repeating units

In the above example of polymerization through functional groups, the single monomer (w-aminoundecanoic acid) contains two types of functional group, namely amino and carboxyl In practice it is more usual to use a

6 BASIC CONCEPTS

mixture of two monomers, each having only one type of functional group; such monomers are generally more readily obtainable This technique is illustrated by the formation of a polyamide (nylon 6,6) from hexamethylene-diamine and adipic acid:

1

[-HN-(CH2)6-NH-OC-(CH2)4-C0 1 + 2nH20

Further examples of polymerizations of this kind are as follows:

hexamethylenediamine sebacic acid

GENERAL METHODS OF PREPARA nON OF POLYMERS 7

It will be noticed that in all the above examples the production of polymer is accompanied by the formation of a secondary product; in each case there is elimination of some small molecule as a by-product, e.g water or hydrogen chloride However, polymerization through functional groups does not al-ways result in such a by-product For example, in the reaction between an isocyanate, such as 1,6-hexamethylene diisocyanate, and a glycol, such as 1,4-butanediol, polymer is the sole product:

nOCN-{CHz)6-NCO + nHO-{CHz)4-0H ->

[-OC-HN-{CHz)6-NH-CO-O-{CHz)4-0-]

The polymer contains urethane groups along the chain and is, therefore, a polyurethane The significance of the absence of by-products in reactions of this kind is referred to later in this chapter (section 1.4.5)

The chemistry involved in polymerizations through functional groups is essentially the chemistry of simple organic reactions wherein the correspond-ing functional groups are present in small molecules These polymerizations nearly always proceed in a stepwise manner and the polymer chain is therefore built up relatively slowly by a sequence of discreet interactions between pairs of functional groups Each interaction is chemically identical since each involves the same kind of functional group It may also be noted that the reactivity of a functional group at the end of a polymer chain is similar to that of the corresponding functional group in a monomer molecule Thus a functional group at the end of a polymer chain can react with a functional group which is either in a monomer molecule or at the end of another polymer chain The growth of a polymer chain is therefore somewhat fitful Since there is no inherent termination reaction, the molecular weight of the polymer continues to increase with time until, ideally, no more functional groups remain available for reaction By far the greatest majority of poly-merizations through functional groups proceed in a stepwise manner but a few have been found to involve a chain reaction (see section 1.4.5)

1.4.2 Polymerization through multiple bonds

This method of polymer preparation may be simply regarded as the joining together of unsaturated molecules through the multiple bonds There is essentially no difference in the relative positions of the atoms in the un-saturated molecules and in the structural units of the polymer and there is no change in composition Polymerizations of this kind may be divided into the various categories which follow

(a) Vinyl polymerization

The most common unsaturated compounds which undergo polymerization through their multiple bonds contain carbon-carbon double bonds and are

CH3 I - l -CH2 tH- " CH3 ]

nH 2C==CH propylene polypropylene

CI 6HS - l -CH2-tH- " C6HS]

nH2C=CH styrene polystyrene

CI

[-CH2-~IH- t

-nH2C==CH vinyl chloride poly(vinyl chloride)

polymeriz-Vinyl polymerization, as illustrated by the above reactions, involves a three-part process, namely initiation, in which is formed an active species capable of starting polymerization of the otherwise unreactive vinyl com-pound; propagation, in which high molecular weight polymer is formed; and

termination, in which deactivation occurs to produce the final stable polymer The active species in vinyl polymerizations may be of three different types,

GENERAL METHODS OF PREPARA nON OF POLYMERS 9

namely free radicals, anions and cations and these possibilities give rise to three distinct methods of accomplishing polymerization

Free radical polymerization

In free radical polymerization, initiation may be brought about by light or heat; most commonly, however, it is achieved by the addition of a material which, on heating, decomposes into free radicals (which may be defined as molecules containing an unpaired electron) Examples of frequently used initiators are benzoyl peroxide and azobisisobutyronitrile which give rise to free radicals as follows:

The initiation of polymerization is therefore a two-step sequence The first step is the dissociation of the initiator, as illustrated above This may be represented as:

10 BASIC CONCEPTS

Propagation continues until the growing long chain radical becomes activated Such termination is commonly by reaction with another long chain radical in one of two ways:

of termination depends on the experimental conditions and the monomer involved Termination of a growing polymer chain may also occur by

formation of a new free radical Transfer reactions have the general form:

-CH2-CHR + AB + -CH2-CHRA + B'

where AB may be monomer, polymer, solvent or added modifier Depending

on its reactivity, the new free radical (Bo) mayor may not initiate the growth

of another polymer chain The transfer of hydrogen from monomer to growing polymer may be given as an example of termination by transfer:

-CH2-CHR + H2C=CHR 4 -CH2-CH2R + H2C=CR

Anionic polymerization

In anionic polymerization of vinyl monomers the active centre is a car bani on Substances which initiate this type of polymerization are of two kinds:

XHya- where the actual or potential anion Y- is able to add to the carbon carbon double bond to form a carbanion which can then propagate Initiators of this kind include alkali metal alkyls, aryls, alkoxides and amides The initiation step which occurs with potassium amide will serve to illustrate the mode of reaction of such initiators:

K +H2~- + H2C=CHR 4 H2N-CH2-~HR K +

(ii) Free metals which are able to transfer an electron to the monomer with the consequent formation of an anion-radical Alkali metals (M 0) are the most common initiators of this type and may initiate polymerization by two different kinds of electron-transfer reactions:

(a) Direct transfer of an electron to the monomer to form the initiating species:

GENERAL METHODS OF PRE PARA nON OF POL YMERS 11

In most solvents the resultant anion-radical rapidly dimerizes to give a dicarbanion which functions as the actual initiating species:

2Hi-~HR M+ -> M+RH~-CH2-CH2-~HR M+

(b) Transfer of an electron to an intermediate compound (A) to form an ion radical which subsequently transfers the electron to the monomer to form the initiating species:

M' + A -> M+

A'-M + A'- + H 2C=CHR -> Hi:-~HR M + + A

The resultant anion-radical generally dimerizes to give a dicarbanion which functions as the actual initiating species (see above) An example of this type

of initiator system is the solution obtained by adding sodium to naphthalene

in an inert solvent such as tetrahydrofuran The reactions involved are:

-CH2-~HR + H 2C=CHR -> -CH2-CHR-CH2-~HR

However, in general the presence of a counter (or gegen) ion in close proximity to the active centre has a profound effect Thus, whilst in free radical polymerization the growth of the propagating chain is independent of the initiator used, the same cannot be said of anionic polymerization In particular, the separation between the carbanion end-group and the counter ion is the primary factor determining the stereochemistry of the propagation reaction Also in contrast to free radical polymerization, true termination reactions are absent from anionic polymerizations Under vigorous reaction conditions the active centre may be destroyed by hydride elimination:

-CH2-~HR M+ -> -CH=C!-lR + MH

A similar reaction may also result in transfer of activity to monomer:

-CH2-~HR M+ + H 2 C=CHR -> -CH=CHR + H3C ~HR M+

12 BASIC CONCEPTS

The presence of a solvent may provide another mode of transfer The mechanism of such a reaction cleafly depends on the nature of the solvent; one possibility is proton transfer trom the solvent (S-H):

-CH2-~HR M+ + S-H -> -CH 2 -CH 2 R + S-M+

Impurities containing active hydrogen also participate in transfers of the above type Carbon dioxide inhibits polymerization by forming a carboxylate anion which is not sufficiently reactive to initiate further polymerization:

GENERAL METHODS OF PREP ARA nON OF POLYMERS 13

generally regarded as involving a series of alkylations and reductive ations of the following kind:

dealkyl-(C2HshAl + TiCl 4 > (C2HshAlCl + C2HsTiCl 3

C2H S TiCl 3 > °C2Hs + TiCl 3 (C2HshAl + TiCl 3 > (C2Hs)2AlCl + C2HsTiCl2

C2HS TiCl2 > °C2Hs + TiCl2 (C2HshAl + C2HS TiCl 3 > (C2Hs)2AlCl + (C2HSh TiCl2

(C2Hsh TiCl2 > °C2Hs + C2Hs TiCl2

Several other alkylation reactions may be written The formation of ethyl free radicals by decomposition of unstable alkyltitanium compounds accounts for the evolution of the hydrocarbon gases Thus the final product is a complex mixture of organo-aluminium and -titanium compounds, lower titanium chlorides and some organic fragments The actual composition of the product

is dependent on the relative proportions of the starting materials and the time and temperature of reaction

It is generally supposed that the active component ofthe catalyst mixture is

a complex formed between titanium trichloride and triethylaluminium This complex is envisaged as forming at the surfaces of titanium trichloride crystals; its formation may be regarded as the strong adsorption of the aluminium compound on to the crystal surfaces By analogy with the bridged structures known to be present in dimeric aluminium alkyls, the complex is assumed to have the following structure:

CL,h/Cl.··· b+, C2HS

b-, TI)I1 b- :Al'-.,

CH 3

In such a complex the titanium has unfilled 3d-orbitals to which may be

co-ordinated n-electrons from the double bond of the vinyl monomer Thus a possible representation of initiation and propagation is as follows:

14 BASIC CONCEPTS

The essential feature of this mechanism is that monomers are inserted, one after the other, into a polarized titanium carbon bond The polymer there-fore grows out of the active centre, rather as a hair grows from the root It will

be noticed that the propagating end of the polymer chain is negatively charged and therefore the reaction may be regarded as an anionic polymeriz-ation Chain growth may be terminated by several types of transfer, e.g

Internal hydride transfer:

Cat-CH z-CHR+CH z-CHR+,CH z-CH3 +

Cat-H + H zC=CRTCH2-CHR+.CH2-CH 3

Transfer to monomer:

Cat-CH 2-CHR+CH 2-CHR+.CH 2-CH 3 + H2C=CHR + Cat-CH 2-CH2R + H2C=CR+CH2-CHR+.CH2-CH 3

True (kinetic) termination may be brought about by the addition of an active hydrogen compound such as an alcohol:

Cat-CH 2-CHR+CH2-CHR+.CH 2-CH3 + R'-OH +

Cat-OR' + H 3C-CHR+CH 2-CHR+.CH 2-CH 3

Cationic polymerization

In cationic (or more specifically, carbo cationic) polymerization of vinyl

monomers the active centre is a carbenium ion Substances which initiate this type of polymerization are of three kinds:

initi-ation step consists of the transfer of a proton to the monomer:

Chain growth continues until either chain transfer or termination occurs In

chain transfer, there is no loss of active centres and the kinetic chain remains operational Various types of chain transfer are possible, e.g

Transfer to counter ion:

A-In true (kinetic) termination there is irreversible loss of propagating ability

This may be brought about ill several ways, the most important of which is

GENERAL METHODS OF PREPARA nON OF POLYMERS 15

neutralization In this process, the propagating carbenium ion and ion interact to give an electrically neutral species With trifluoracetic acid, for example, termination occurs by ester formation:

(iii) Metal halides of the type which catalyses the Friedel-Crafts reaction, e.g aluminium trichloride, boron trifluoride and ferric chloride The pure, an-hydrous metal halides do not initiate polymerization; they are active only in the presence of co-initiators Co-initiators are commonly compounds containing active hydrogen, e.g alcohols, protic acids and water: The co-initiator (QH) co-ordinates with the metal halide (MX.) to form a complex protic acid which transfers a proton to the monomer:

MX + QH -+ [QMX.rW

+

[QMX.rW + H 2 C=CHR -+ H3C-CHR [QMX.r

Propagation then proceeds as follows:

Chain transfer takes place as discussed above The nature of termination reactions has not been fully established The advantage of the metal halide initiators over protic acid initiators is their ability to extend the lifetime of the kinetic chain and thus give polymers of higher molecular weight For commercial purposes, the metal halides are by far the most important type of initiators used for cationic polymerization

By way of conclusion to this short discussion of vinyl polymerization, it may be noted that not all vinyl monomers can be polymerized to high molecular weight polymers by all three of the general methods described, namely free radical, anionic and cationic polymerization Table 1.1 indicates the general applicability of the three methods in homogeneous systems to some vinyl monomers Often the effectiveness, or otherwise, of the methods can be related to the polarity of the monomer double bond Electron-releasing substituents favour the formation of carbenium ions and render the monomer susceptible to cationic polymerization Thus isobutene (with two electron-releasing methyl groups) and vinyl ethers (in which the resonance

16 BASIC CONCEPTS

Table 1.1 General applicability of polymerization methods in homogeneous systems

to some vinyl monomers

Monomers are arranged in approximate order of increasing susceptibility to anionic merization

poly-effect, due to delocalization of an un shared electron pair on the oxygen atom, outweighs the inductive effect exerted by the ether group to give an overall electron-releasing effect) undergo cationic polymerization exclusively On the other hand, electron-withdrawing substituents favour the formation of car-banions and render the monomer susceptible to anionic polymerization Thus vinylidene cyanide (with two electron-withdrawing cyano-groups) and I-nitro-l-alkenes (in which the resonance and inductive effects exerted by the nitro-group reinforce one another and outweigh the inductive effect exerted

by the alkyl group to give an overall electron-withdrawing effect) undergo anionic polymerization exclusively Between these two extremes lie those monomers wherein the electron-releasing and -withdrawing effects are less pronounced; these monomers undergo free-radical polymerization and show

a tendency to undergo either cationic or anionic polymerization, depending

on the nature of the substituent A few monomers such as styrene (in which the phenyl group can function as either an electron source or electron sink) can be polymerized by all three methods It may be noted here that the fact that a monomer has a high tendency to polymerize does not necessarily mean

it forms high molecular weight polymer The molecular weight is also governed by factors such as transfer and termination reactions

(b) Diene polymerization

Conjugated dienes comprise the second group of unsaturated compounds which undergo polymerization through their multiple bonds The most

GENERAL METHODS OF PREPARATION OF FOLYMERS 17

common dienes used for the preparation of commercially important mers are butadiene, chloroprene and isoprene These are normally represen-ted by the following structures:

poly-H2C=CH-CH=CH2 butadiene

Cl

I

H2C=C-CH=CH2 chloroprene

CH) H2C=t-CH=CH2 isoprene

Such monomers can give rise to polymers which contain various isomeric structural units Each of the above structures contains a 1,2- and a 3,4-double bond and there is thus the possibility that either double bond may participate independently in polymerization, giving rise to I,2-units and 3,4-units re-spectively:

x

I -CH -C-

With symmetrical dienes such as butadiene, these two units become identical

A further possibility is that both bonds are involved in polymerization through conjugate reactions, giving rise to l,4-units A l,4-unit may occur as either the cis- or the trans-isomer:

-CH 2, /CH C=C

-CH 2, /H

_./C=C,

x CH

Generally speaking, the polymer obtained from a conjugated diene tains more than one of the above structural units The relative frequency of each type of unit is governed by the nature of the initiator and the experi-mental conditions as well as the structure of the diene Each of the three general methods of accomplishing vinyl polymerization, described above, may be used for the polymerization of conjugated dienes

con-In free radical polymerization, the various structural units may be aged as arising from the addition of one or other end of a monomer molecule

envis-to one of the resonant forms of the growing polymeric radical, e.g

With regard to anionic polymerization of conjugated dienes, alkali metal alkyls and free alkali metals are the most commonly used initiators The resultant polymers generally have much higher contents of 3,4-units com-pared to the polymers prepared by free,radical polymerization When an alkali metal alkyl (M+R -) is the initiator, the initiation reaction may be represented as:

f

M+R- + H2C=CHC=CH2

-GENERAL METHODS OF PRE PARA TION OF POLYMERS 19

Addition of a monomer molecule to one of the resonant forms of the anion then results in the formation of the corresponding structural unit When an alkali metal (Mo) is the initiator, the initiation reaction may be represented as:

Polymerization then proceeds in the manner of the alkyl-initiated reaction described above It may be noted here that lithium (both as the free metal and

as alkyls) stands in marked contrast to the other alkali metals in giving rise to polymers with high proportions of l,4-units This is attributable to the covalent character of the lithium carbon bond, as is discussed later (Chapter 20)

The use of Ziegler-Natta catalysts in the polymerization of conjugated dienes has been widely investigated It is characteristic of these catalysts that the resulting polymers often contain a very high proportion of one type of structural unit By appropriate choice of catalyst, polydienes comprised almost exclusively of cis-I,4-, trans-I,4-, 1,2-, and 3,4-units have been ob-tained The mechanisms of such reactions are, at present, somewhat obscure but presumably the diene molecule co-ordinates with the metal carbon bond

of the catalyst in a manner similar to that involving the lithium carbon bond mentioned above

In contrast to free radical and anionic polymerization, the cationic merization of conjugated dienes has received little study Generally, rather low molecular weight polymers are produced and these have not attained commercial significance A feature ofthe products obtained by the polymeriz-ation of conjugated dienes using metal halide catalysts is that they contain an appreciable proportion of cyclized structures The linear portions of the polymers consist mainly of trans-I,4-units

poly-The profound effect of the initiator on the microstructure of the products obtained by polymerization of conjugated dienes is illustrated by Table 1.2

20 BASIC CONCEPTS

Table 1.2 Effect of initiator on microstructure of polyisoprene [1]

Initiator Structural units (%)

(c) Hetero-multiple bond polymerization

In both the above types of polymerization through multiple bonds, namely vinyl and diene polymerization, a carbon-carbon double bond is the active site However, multiple bonds involving elements besides carbon may also be utilized in the preparation of polymers, which then contain hetero-atoms in the main chain The most common unsaturated monomers used for the preparation of polymers of this kind are carbonyl compounds Formaldehyde has been the most widely studied in this respect and its polYPlers are of commercial importance The product of polymerization of formaldehyde may be regarded as a polyether:

nH 2 C=O -> [-CH2-0-J

and is discussed in Chapter 9 As will be shown, polymerization may be accomplished by the use of both anionic and cationic initiators, but free radical initiators are ineffective

A further example of this category of polymerization is the polymerization

of monoisocyanates through the carbon-nitrogen double bond This merization is carried out at low temperatures (to limit cyclization) with an anionic initiator, e.g sodium cyanide:

GENERAL METHODS OF PREPARATION OF POLYMERS 21

1.4.3 Polymerization through ring-opening

Many cyclic compounds undergo ring-opening reactions which lead to polymer formation Usually, the structural units of polymers prepared in this way have the same composition as the monomer and there is essentially no change in the relative positions of the atoms Examples of cyclic compounds which have been found to undergo polymerization are as follows: N-carboxy-oc-aminoacid anhydrides (Leuchs' anhydrides), cyclic ethers, cyclic imines, cyclic sulphides, cyclopropanes, lactams and lactones It will be apparent from this list that cyclic compounds which polymerize usually contain at least one hetero-atom Normally, polymerizations through ring-opening are ac-complished by use of either anionic or cationic initiators Despite the large numbers of cyclic compounds which have been investigated as monomers, commercial importance has been achieved by only two types, namely cyclic ethers and lactams The polymerization of each of these classes of monomer is illustrated by the following examples:

ethylene oxide polyether (poly(ethylene oxide))

H2S::-C,H2 nH2c' FO [-(CH2), -{:O NH-]

H2tyNH H2

caprolactam polyamide (nylon 6)

These types of polymerization are considered in detail in later discussions of polyethers (Chapter 9) and polyamides (Chapter 10)

1.4.4 Polymer modification

For completeness, this fourth general method of preparing polymers is included at this point although it cannot be regarded as a method of polymerization This technique consists simply of subjecting an existing polymer to such chemical reaction that a different polymer is obtained For example, poly(vinyl acetate) may be subjected to alcoholysis by treatment with methanol to give poly(vinyl alcohol):

1.4.5 Classification of polymerization reactions

In 1929 Carothers made the proposition that all polymers could be divided into two types, namely condensation polymers and addition polymers A condensation polymer was defined as a polymer in which the structural unit contains fewer atoms than the monomer (or monomers) from which the polymer is derived An addition polymer was defined as a polymer in which the structural unit has the same molecular formula as the monomer A limitation of this classification is that some polymers may be included in both categories For example, polyethylene is usually prepared by the polymeriz-ation of ethylene:

The polymer may thus be counted as an addition polymer A polymer which may be considered to be the same (if any differences in molecular weight are disregarded) can be formed from decamethylene bromide and sodium in the Wurtz reaction:

nBr(CH~)10Br + 2nNa -+ [-CHz-CHz-]sn + 2nNaBr

On the basis of this reaction, polyethylene may be counted as a condensation polymer

The foregoing proposals by Carothers are also commonly used as the basis

of a scheme of classification of polymerization reactions In this scheme, the processes by which polymers are formed are divided into condensation

poly-merization leads to a polymer in which the structural unit contains fewer atoms than the monomer whilst an addition polymerization results in a polymer having a structural unit with the same molecular formula as the monomer A limitation of this classification is that a somewhat anomalous situation arises when polymerization through functional groups (see section 1.4.1) is considered If there is elimination of a by-product then clearly

GENERAL METHODS OF PREPARA nON OF POL YMERS 23

the reaction counts as a condensation polymerization but if no by-product is evolved then the reaction must be regarded as an addition polymerization Since whether or not the structural unit differs in composition from the monomer is of no particular significance it is not very desirable so to separate reactions which are similar in all other respects Many authors therefore apply the term condensation polymerization to polymerization through functional groups irrespective of whether or not a by-product is formed Some authors, however, prefer to restrict the term to only those cases where there is elimination of a small molecule: they then apply the term rearrange-

proceeds through the interaction of functional groups without elimination of

a small molecule Addition polymerization has been defined above as leading

to a polymer with a structural unit having the same molecular formula as the monomer On this basis, all polymerizations through multiple bonds are to

be classified as addition polymerizations Generally polymerizations through ring-opening also come into this category, but the polymerization of N-

carboxy-a-aminoacid anhydrides may be cited as an exception since a product is formed:

by-R c -C~

n 2 I 0 _ [-NH-CR2-CO-] + nC0 2

HN cb

An alternative method of classifying polymerization reactions is according

to mechanism In this scheme, processes are divided into stepwise

built up relatively slowly by a sequence of discrete reactions; the initiation, propagation and termination reactions are essentially similar, each having the same rate and mechanism A feature of this type of polymerization is that

a monomer molecule is capable of reacting with another monomer molecule

or with a polymer molecule with equal facility As a result there is rapid disappearance of monomer at an early stage in the reaction but a high reaction conversion is required for the attainment of high molecular weight

It has been shown previously that polymerizations through functional groups generally proceed in a stepwise manner As an exception to this rule may be cited the polymerization of benzyl chlorides, e.g 2,5-dimethylbenzyl chloride:

+ nHC)

Although polymerization involves functIOnal groups (phenyl and methyl) and a by-product is eliminated, a stepwise reaction does not occur In

chloro-24 BASIC CONCEPTS

fact, polymerization proceeds through a chain reaction involving cationic species (Catalytic quantities of metal halides initiate the polymerization.) In contrast to stepwise polymerization, chain polymerization involves a chain reaction in the kinetic as well as the structural sense and a polymer molecule grows extremely rapidly once initiation has occurred Typically, several thousand structural units are added in the space of approximately one second The initiation, propagation and termination reactions are signifi-cantly different in rate and mechanism In this case a monomer molecule cannot react with another monomer molecule but only with an active end-group on a polymer radical or ion Thus high molecular weight polymer and monomer are present throughout the reaction As has been shown previously, most polymerizations through multiple bonds and through ring-opening involve a chain reaction By way of exception to this rule may be cited polymerization based on the Diels-Alder reaction; polymerization is through multiple bonds but proceeds in a stepwise manner, e.g

In this example 2-vinylbutadiene acts as both the diene and the dienophile to give a palycyclohexene (For clarity, only one of the possible isomeric structural units is shown.)

1.5 POL YMERIZA nON TECHNIQUES

In principle, a polymerization reaction may be carried out in the solid phase, the liquid phase or the gas phase In practice, commercial scale polymeriz-ations are almost always conducted in the liquid phase It may be noted, however, that gas phase processes for polyolefins are of importance (See Chapter 2.) Liquid phase polymerizations may be subdivided into four types according to the nature of the physical system employed All of these variations find widespread application throughout the polymerization methods discussed in the previous section, as is illustrated by Table 1.3 which gives the techniques commonly used in the production of some commercial polymers

mono-mer/polymer This technique is most commonly used for polymerizations which proceed through functional groups in a stepwise manner and then the method merely involves heating the straight monomer or monomer mixture (sometimes with addition of a small amount of a catalyst to increase the reaction rate) The system is maintained in a fluid state by keeping the

POLYMERIZATION TECHNIQUES Table 1.3 Polymerization techniques used in the production of

some commercial polymers

Polyethylene (low density)

Polyethylene (high density)

Solution Bulk, Suspension Solution Solution Emulsion Suspension Solution Emulsion Solution Bulk

25

temperature sufficiently high As has been mentioned previously, in this type

of reaction there is a progressive increase in molecular weight and the high viscosity of the resultant polymer melt can lead to handling difficulties When the technique of bulk polymerization is applied to polymerizations which involve chain reactions, the straight monomer is heated with a small amount

of appropriate initiator Again there is a substantial rise in viscosity as the concentration of polymer (which is soluble in the monomer) increases and this can lead to difficulty in dissipating the high exothermic heat of reaction which is usually a feature of such polymerizations Because of the possibility

of localized overheating leading to degradation and discoloration of the polymer, bulk polymerization is seldom practised with large batches It may

be noted that bulk polymerization results in relatively pure polymer

polymerization This technique is commonly employed for the ionic merization of gaseous vinyl monomers The solvent facilitates contact of monomer and initiator (which mayor may not be soluble in the solvent) and assists dissipation of exothermic heat of reaction A limitation of this tech-nique is the possibility of chain transfer to the solvent with consequent formation of low mwecular weight polymer An added disadvantage is the need to remove the solvent in order to isolate the solid polymer In this respect, it is common practice to use a solvent in which the monomer but not the polymer is soluble; the polymer is then obtained directly as a slurry and little further purification is necessary

poly-26 BASIC CONCEPTS

droplets (generally about 10-2-10-1 cm in diameter) maintained by vigorous slirring This technique is extensively used for the free radical polymerization

of vinyl monomers A monomer-soluble initiator is added and ation occurs within each droplet Generally a material such as poly(vinyl alcohol) or gelatin is added to provide a protective coating for the droplets; this prevents the droplets from cohering when they are at the stage of being composed of a sticky mixture of monomer and polymer Besides facilitating the removal of exothermic heat of reaction, suspension polymerization has the advantage that the polymer is obtained in the form of small beads which are easily collected and dried The polymer is relatively free from contami-nants and there are no solvent recovery considerations

soap (usually about 5%) to form an emulsion; such a dispersion is stable and its existence is not dependent on continued agitation This technique is extensively used for the free radical polymerization of diene monomers in the preparation of synthetic rubbers In this case a water-soluble initiator is used and the course of the polymerization is considerably different from that followed in the systems described previously At the start of an emulsion polymerization three components are present:

(i) Relatively large droplets of monomer, about 10-4 cm in diameter, ized by soap molecules around the periphery

stabil-(ii) Aggregates (micelles) of 50 100 soap molecules swollen with monomer to

in which case termination quickly occurs because of the small volume of the reaction locus The micelle then remains inactive until a third free radical enters, and so on As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules around the periphery Monomer continues to diffuse into these particles and polymerization is maintained therein until the monomer supply is exhausted The final product is a stable dispersion (latex)

POL YMER STRUCTURE 27

of polymer particles with diameter of about 10-5-10-4 cm The polymer is isolated by 'breaking' the latex, usually by the addition of acid which converts the soap to fatty acid In some instances, the latex is used directly without coagulation; such is the case in, for example, the preparation of poly(vinyl acetate) latex paints An attractive feature of emulsion polymerization is that

it is possible to prepare very high molecular weight material at high rates of conversion A limitation of the method is the difficulty of washing the product free of soap residues which impair the electrical insulation properties and optical clarity

1.6 POLYMER STRUCTURE

Up to this point, polymers have been considered simply as large molecules composed of repeating structural units and there has been little consideration

another Several variations are possible and these can have pronounced effects on the properties of a polymeric material The more important of these structural arrangements may be conveniently considered under the four headings which now follow

1.6.1 Polymer geometry

In most of the examples given so far, the polymer molecules have been linear,

that is to say, the molecules have a thread-like shape This description may be misleading since it may be taken to mean that these molecules resemble a thread in its most commonly encountered form, i.e in a more or less fully extended state In fact, linear polymer molecules may have various conform-ations In amorphous materials the molecules are highly kinked in an irregular fashion (randomly coiled) (see Fig 1.2a) whilst in crystalline mater-ials the molecules are generally spiral or zig-zag The point is that a thread could be made to resemble these conformations

Not all polymers, however, are linear; various non-linear forms are also possible In branched polymers, linearity is destroyed by the presence of side-chains Branched polymers commonly arise in two types of polymerization process:

discussion of free radical vinyl polymerization it was mentioned that various transfer reactions may occur One possibility is the transfer of hydrogen from within a polymer to a growing polymer, e.g

-CH 2 -CHR- + -CH 2 -CHR -> -CH 2 -CR- + -CH 2 CH 2 R

28 BASIC CONCEPTS

Fig 1.2 Planar representation of polymer molecules (a) Randomly coiled linear polymer (b) Slightly branched polymer (c) Highly branched polymer (d) Cross-linked polymer with 3-functional junctions as might arise by continued reaction of (c) (e) Cross-linked polymer with 4-functional junctions, as might arise by cross-linking

an unsaturated linear polymer

The resultant free radical may initiate polymerization of monomer, in which case a branch is produced:

-CR CH 2 -CR- + H 2 C=CHR -+ I

CH 2 -CHR etc

In general, this type of branching arises when the propagating radical is

highly energetic (i.e lacking in resonance stabilization) and the polymer contains readily replaceable hydrogen atoms (i.e the resultant free radical is resonance stabilized) Thus branching occurs in, for example, polyacrylates and poly(vinyl acetate) (see Chapters 6 and 5) Similarly, branching also occurs in free radical diene polymerization As was mentioned previously, the polydienes contain 1,2- and 3,4- units These units contain pendant vinyl groups which are susceptible to free radical attack and consequent branching; these units also contain allylic hydrogen atoms which are readily abstracted

to give sites for branching The branched polymers obtained in vinyl and diene polymerization may generally be regarded essentially as linear poly-mers with relatively few side-chains (see Fig 1.2b)

POL YMER STRUCTURE 29 (ii) Polymerization of poly functional monomers In the previous discussion of polymerization through functional groups, the reaction between a diisocyan-ate and a glycol was seen to result in a linear polyurethane If, however, a triisocyanate and a glycol are used the resultant polyurethane is branched; the reaction may be represented as follows:

l NH-CO-O-R' -OH

I NH-CO-O-R'-O-CO-NH-R-NCO

I OCN-R-NH-CO-O-R'-OH etc

In this type of reaction the product is highly branched (see Fig 1.2c) and cross-linking (see later) occurs before a high molecular weight can be reached

It will be apparent from this example that in polymerization through tional groups, the functionality of the reactants determines whether a linear or branched polymer is obtained The functionality of a compound is the number of groups per molecule which are capable of undergoing the poly-merization reaction If all the reacting molecules are difunctional a linear polymer results; if polyfunctional (i.e trifunctional or greater) molecules are present branching occurs It may be noted that these comments regarding functionality are, in essence, applicable to polymerization through multiple bonds if each multiple bond is regarded as being difunctional Thus vinyl compounds, conjugated dienes (where normally only one double bond is involved) and carbonyl compounds are designated as difunctional As has been seen, these monomers give rise to essentially linear polymers (although subsequent transfer reactions may lead to some branching) Likewise, cyclic monomers are regarded as difunctional

func-The final structural form to be described in this section relates to

ways:

de-scribed above which involves a triisocyanate and a glycol, the branched structures initially formed contain reactive groups, i.e isocyanate and hy-droxyl groups Thus if reaction is allowed to proceed, the branched structures

30 BASIC CONCEPTS

link up with one another to form large continuous three-dimensional work (see Fig 1.2d) Materials such as epoxy, phenol-formaldehyde and silicone resins are utilized commercially in this way

net-(ii) Cross-linking of linear polymers It was seen above that some branching occurs during the polymerization of conjugated dienes and some vinyl monomers At sufficiently high degrees of conversion, continued reaction may result in the formation of cross-links For example, the combination of two growing branches on two different polymer chains would result in a cross-link, e.g

-CH 2

-CR-I

CH 2

I CHR CHR

1.6.2 Structural unit variety

All the polymers considered so far have been homopolymers, that is, each polymer has consisted of repeating structural units of the same kind (dis-regarding terminal units) It is, however, possible to prepare polymers which contain more than one type of structural unit; such polymers are dis-tinguished by the term copolymer Most important commercial copolymers contain two kinds of structural unit whilst a few have three units (The latter products are often called terpolymers.) Copolymers may be prepared by the same general methods described above for homopolymers but, of course,

POLYMER STRUCTURE 31

more than one monomer must be used Depending on the monomers chosen and the experimental techniques used, various distributions of structural units within the polymer chain may be achieved Various possible arrange-ments of the two structural units A and B are shown in Fig 1.3

Few truly alternating copolymers have been prepared; the best known example is the product obtained by the free radical copolymerization of an approximately equimolar mixture of maleic anhydride and styrene There is virtually no tendency for the radical end of a growing chain to react with its own type of monomer The structure of the product is therefore essentially as follows:

It may be noted that this structure could be regarded as a homopolymer having only the following structural unit:

32 BASIC CONCEPTS

However, it is usual to consider as copolymers products obtained from a mixture of monomers when each of the monomers is separately capable of forming a homopolymer (under appropriate conditions) Thus such polymers

as polyamides derived from diamines and dibasic acids are not counted as alternating copolymers since the monomers are not separately polymerizable Block copolymers can be prepared by several techniques, of which anionic polymerization offers the best possibilities for controlling the product In this method the first step is to polymerize a single monomer, allowing reaction to proceed until the monomer is exhausted To the 'living polymer' is added a second monomer which then forms the second block When the second monomer is exhausted a third monomer may be added, and so on Many combinations of monomers have been investigated and a few block copoly-mers are now commercially available, e.g the styrene-butadiene copolymer described in Chapter 20

Graft copolymers may be prepared in three general ways, namely transfer grafting, irradiation grafting and chemical grafting Transfer grafting is most commonly free radical initiated Typically, a vinyl or diene polymer is treated with a peroxide in the presence of vinyl monomer Transfer occurs between the polymer chain and radicals derived from the initiator; the resultant polymer chain radical then initiates polymerization of the monomer, e.g

+ R'O' -> -CH 2 -CR,- + R'OH

[CH 2 -CHR"-J

I + nH 2 C=CHR" -> -CH 2 -CR-

Grafting is invariably accompanied by formation of homopolymer of the monomer to be grafted In irradiation grafting, an essentially similar process

is involved except that the reactive sites on the polymeric substrate are created by irradiation (commonly with ultraviolet light) In chemical grafting, reactive groups present along the polymer chain are used as sites for grafting Both free radical and ionic reactions have been utilized in this technique One method involves irradiation of the polymeric substrate in the presence of oxygen to produce peroxide groups which can be subsequently decomposed thermally in the presence of monomer to initiate free radical grafting, e.g

90H -CH 2 -CHR - + O 2 - -CH 2 -CR-

An example of ionic grafting is provided by the formation of acrylonitrile) from poly(p-lithiostyrene) (prepared by treatment of poly(p-

poly(styrene-g-iodostyrene) with n-butyllithium);

POLYMER STRUCTURE 33

BuLi

At the present time, graft copolymers have achieved limited commercial importance

1.6.3 Structural unit orientation

A further possible variable in polymer structure arises when vinyl and diene monomers are polymerized

In vinyl polymerization, three different types of linkage may be formed since each monomer molecule can assume either of two orientations as it adds to the preceding unit:

The linkages are identified by referring to the substituted end of the monomer molecule as the 'head' and to the unsubstituted end as the 'tail' In the above scheme, the tail-to-head linkage is equivalent to the head-to-tail linkage Thus, in theory, three arrangements are possible in the polymer; the linkages may be all head-to-tail, they may be alternately head-to-head and tail-to-tail,

or they may be mixed:

-CH2-CHR-CH2-CHR-CH2-CHR-CH2-CHR-CH2-CHR-head-to-tail polymer -CH2-CHR-CHR-CH2-CH2-CHR-CHR-CH2-CH2-CHR-

head-to-head/tail-to-tail polymer -CH2-CHR-CH2-CHR-CHR-CH2-CHR-CH2-CH2-CHR-

random polymer

It follows that a polymer which contains head-to-head linkages must also contain tail-to-taillinkages and vice versa All the vinyl polymers which have been examined so far are very predominantly head-to-tail in their orientation The methods which have been used to investigate the structures of specific polymers are described later in the chapters relating to these polymers

34 BASIC CONCEPTS

The orientation of monomer addition is influenced by the following factors:

first step in free radical vinyl polymerization is the homolytic dissociation of the initiator, which may be represented as:

Hi-cHR-I

(1)

(2)

In general, the radical formed in reaction (1) is more stable than that formed

in reaction (2) because in this radical the substituent R is able to make the more effective contribution to resonance stabilization The extent of such stabilization depends on the facility with which R can delocalize the odd electron; the effect is particularly marked when R contains suitably placed multiple bonds The resonance stabilization of radicals derived from some common vinyl monomers is illustrated below:

- + I· + H2C=CH-CI - I-CH2-CH-CI - I-CH2-~H-CI' vinyl chloride